1. Field of the Invention
The present invention relates to an apparatus and method for generating high voltage based on digital control. More particularly, the present invention relates to a high voltage generating apparatus that can generate high voltage by detecting the optimal switching time point, and a high voltage generating method thereof.
2. Description of the Related Art
An image forming device is an apparatus for printing an image corresponding to an inputted original image data on a recording medium such as print paper. Examples of the image forming device include a printer, a photocopier, a facsimile and the like. An electrophotographic method used in the image forming device is adopted by a laser beam printer, a light emitting diode (LED) print head printer, a facsimile and other image forming devices. Electrophotographic image forming devices perform printing operation through charging, light exposure, development, transfer, and fixing.
FIG. 1 is a cross-sectional view showing a conventional electrophotographic image forming device. Referring to FIG. 1, the conventional electrophotographic image forming device includes a photosensitive drum 1, a charging roller 2, a laser scanning unit (LSU) 3, a developing roller 4, a transfer roller 5, a controller 6, and a high-voltage power supply (HVPS) 70.
The printing operation of the electrophotographic image forming device having the above structure will be described hereafter. First, the high-voltage power supply 70 supplies a predetermine level of voltage to the charging roller 2, the developing roller 4 and the transfer roller 5 under the control of the controller 6. The charging roller 2 charges the surface of the photosensitive drum 1 uniformly with the charging voltage supplied from the high-voltage power supply 70. The laser scanning unit 3 scans the photosensitive drum 1 with light corresponding to image data inputted from the controller 6 to thereby form an electrostatic latent image on the surface of the photosensitive drum 1.
Subsequently, a toner image is formed of toner supplied by the developing roller 4 in the electrostatic latent image on the surface of the photosensitive drum 1. The transfer roller 5 driven by a transfer voltage supplied by the high-voltage power supply 70 transfers the toner image formed on the photosensitive drum 1 onto paper for recording. The toner image transferred onto the print paper is fixed on the print paper by heat and pressure supplied from a fixer (not shown) and then gets on an ejection path (not shown) to be ejected outside finally.
As described above, the high voltage generating apparatus 70, which is an essential element of a photocopier, a laser beam printer and a facsimile, transforms a low voltage ranging from 12 to 24V up to high voltage ranging from hundreds to thousands volts in a moment and makes it possible to perform printing by forming high-voltage discharge on the photosensitive drum of a printer or a photocopier. The high voltage generating apparatus 70 senses voltage or current and used as a constant voltage source or a constant current source according to purpose of usage.
FIG. 2 is an exemplary circuit diagram showing a conventional high voltage generating apparatus. Referring to FIG. 2, the conventional high voltage generating apparatus includes a low-pass filter 10, a voltage controller 20, a P2 oscillator, a power transformation unit 30, a voltage distributor 40, a voltage sensor 50, and a protector 60.
When a pulse width modulation (PWM) signal D(t) for determining a level of output voltage based on a duty ratio is inputted from an engine controlling unit, the low-pass filter 10 transforms the inputted signal into a DC signal through an RC two-step filter. The DC signal is used as a reference signal for controlling the output voltage.
The voltage controller 20 functions as a difference circuit and a controller for amplifying an error signal. It compares the DC signal outputted through the low-pass filter 10 with a signal obtained by performing feedback on an actual output voltage and generates a driving signal of a transistor Q in the oscillator and power transformation unit 30.
The oscillator and power transformation unit 30 control the base current quantity of the transistor Q based on the output signal of the voltage controller 20 to thereby fluctuate the voltages in an emitter and a collector of a transistor. In the voltage fluctuation, the voltage of a first side coil in the power transformer is changed and thus voltage is generated in a second side coil of the power transformer having a high turn rate.
The voltage distributor 40 generates a final direct current high voltage from the alternating current voltage generated in the second side coil of the power transformer by using rectifying diodes D1 and D2 and voltage distributing and planarizing capacitors C4 and C5. The voltage sensor 50 and the protector 60 sense actual output voltage, and generate and transmit a feedback signal to the voltage controller 20 to thereby protect the application of an abnormal level of voltage.
FIG. 2 shows a circuit generating high voltage in a development unit of a particular channel in a high-voltage generating apparatus. Different channels are needed to apply a predetermined level of high voltage to the charging roller 2, the developing roller 4, and the transfer roller 5, individually.
However, the conventional high voltage generating apparatus uses an analog control method to precisely control output of each channel individually and thus an error originated from characteristics variance of parts including an RC filter and a voltage controlling part should be corrected.
Also, since many parts are used, it is hard to decrease costs, and defect in an individual part caused by an external factor may lead to malfunction of the entire apparatus. Also, a transistor used as a switch in the oscillator and power transformation unit 30 is operated in a linear area, the transistor always is heated.
In addition, as illustrated in FIG. 2, the conventional voltage generating apparatus uses many parts, it takes a long time to transport and assemble the parts and space for printed circuit board (PCB) on which the parts are to be placed. Since the parts are fixed on the PCB, it is not easy to control the output voltage.
In order to overcome the above-mentioned problems, the conventional high voltage generating apparatus adopts a digital controlling method. However, the High-Voltage Power Supply (HVPS) obtained by making an analog device into an application-specific integrated circuit (ASIC) typically includes a MOSFET and, when the MOSFET performs switching operation in the linear area of the transistor, the switch can be damaged.